Method and apparatus for dielectric heating

ABSTRACT

A method and apparatus for heating plastics or rubber articles of complex shape in which the articles are moved on a conveyor belt through a hollow waveguide excited with a microwave energy flow or field acting in one direction. A sinuous hollow waveguide perpendicularly communicates and intersects with the first waveguide at a plurality of spaced apart intervals and is excited with the microwave energy flow acting in a direction perpendicular to the first direction. The articles are thus exposed alternately to staggered energy flows acting perpendicularly of one another, whereby to improve the efficiency of heating and avoid local hot spots.

United States Patent Muller et al.

, 14 1 May 30, 1972 [54] METHOD AND APPARATUS FOR DIELECTRIC HEATING [72] Inventors: Paul C. Muller, Felsenau, Switzerland;

' Karl 11. Schneider, Opladen, Germany [73] Assignee: Messrs. Paul Troester Maschinenbau, Hannover-Wulfel, Germany [22] Filed: Apr. 24, 1970 121 Appl. No.: 31,603

30' Foreign Application Priorlty Data Apr. 25, 1969 Germany P 19 21 065.2

[52] us. c1 ..219/10.ss 51 1111.01. ..H05b 5/00 58 Field of Search ..219/1o.ss

[56] References Cited UNITED STATES PATENTS 3,177,333 4/1965 Lamb ..219/1o.s5

3,304,399 2/1967 Timmermans et a1. ..2l9/10.5$ 3,402,277 9/1968 Muller ..2 l 9/l0.55 3,478,900 11/1969 Jeppsar ..2l9/l0.55 3,491,457 l/1970 Schreiber et al.. .....219/10.55 3,500,012 3/1970 Hilton ..2l9/l0.55

Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney-Oberlin, Maky, Donnelly & Renner ABSTRACT A method and apparatus for heating plastics or rubber articles of complex shape in which the articles are moved on a conveyor belt through a hollow waveguide excited with a microwave energy flow or field acting in one direction. A sinuous hollow waveguide perpendicularly communicates and intersects with the first waveguide at a plurality of spaced apart intervals and is excited with the microwave energy flow acting in a direction perpendicular to the first direction. The articles are thus exposed alternately to staggered energy flows acting perpendicularly of one another, whereby to improve the efficiency of heating and avoid local hot spots.

10 Claims, 3 Drawing Figures Patented May 30, 1972 3,666,905

2 Sheets-Sheet 8 METHOD AND APPARATUS FOR DIELECTRIC HEATING This invention relates to methods and'apparatus for the continuous dielectric heating of articles of plastics, rubber or like materials, by the use of micro-wave energy.

Applications of the invention lie, for instance, in the continuous vulcanization of extruded or pressed strands of plastics or rubber, of the most widely differing cross sections,

and in the hardening of wound strands of polyester glass fiber,

for example. 7

Heating apparatus has been previously proposed in which either a rectangular cross-section hollow waveguide is used, with H -Mode excitation in the transversal direction of the objects to be heated, or else a circular section hollow waveguide is used with E-Mode excitation in the longitudinal direction of the objects. It has been found that only articles having particular preselected surface profiles can be treated with these devices in a manner ensuring satisfactory quality. Low degress of efficiency, uneven heating, with so-called snaking, i.e., with thick and thin portions alternating along the objects, are liable to occur owing to focusing of energy in the heated object are the undesirable phenomena attendant upon the operation of such devices with their high field intensities acting in one direction.

An object'of the present invention is to mitigate the above drawbacks and to provide for the dielectric heating of objects of complicated and/or irregular shapes, for example so that objects with irregular cross-sectional areas and/or of varying thicknesses can be heated and vulcanized or hardened rapidly and evenly withoutthe risk of local overheating.

According to the present invention in one aspect there is provided a method of the continuous dielectric heating of articles of plastics, rubber or like dielectric materials by the use of microwave energy comprising exposing an article to be heated successively to at least two spaced apart flows of microwave energy acting at different angles to one another.

' According to the invention in another aspect there is provided apparatus for the continuous dielectric heating of articles of plastics, rubber or like dielectric material, said apparatus comprising a hollow waveguide, a conveyor extending in said waveguide and adapted to move articles to be heated through said waveguide, and at least one further hollow waveguide communicating at an angle with said waveguide,

and means for exciting said waveguides with microwave enerr I An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG.- 1 is a side view of a dielectric heating apparatus according to the invention,

FIG. 2 is a cross section along the line IIII of FIG. 1, and

FIG. 3 is a partial illustration of a self-stressing Teflon lining inserted in the guide channel for the conveyor belt.

The illustrated apparatus comprises two hollow waveguides l and 2, which take sinuous course and which are made up of waveguide conductors (H -Mode) of rectangular cross section. The waveguides l and 2 have substantially parallel straight portions alternating with U-shaped portions. The straight portions are interconnected substantially centrally of their lengths by hollow intermediate waveguides 3 with E- Mode excitation. Guides 3 may be of circular cross section, for example, and are preferably positioned perpendicularly to the straight portions of conductors land 2 so that the straight portions communicate at right angles into the composite waveguide which consists of the series of mutually aligned intermediate pieces 3 together with a central piece 4 and end pieces 5. Central piece 4 is square in cross-section and serves as a resonant cavity, and end pieces 5 are constructed as wave traps.

A conveyor belt 6 made of Teflon is guided to run longitudinally through the composite waveguide 3, 4 and 5. The conveyor belt 6 is tensioned by tensioning and driving pulleys 7 and 8. Within the composite waveguide the upper run of the belt 6 is supported on rollers 9 which are adjustable in height,

The rollers 9 are made, for example, of ceramic or other insulating material and can be adjusted in their vertical positions in unison or individually.

Water loads" 10 and 11 communicate, through flexible connecting tubes, with the outer ends of the two runs of hollow conductor 1 and 2 respectively, and absorb the microwave energy not absorbed until then, so that no-micro-wave emerge from the apparatus.

A hollow U-shaped waveguide 12, of rectangular cross section, is mounted in telescopic sliding contact to the adjacent ends of the runs of waveguides 1 and 2. The relative positions of waveguide 12 on the runs ofwaveguide 1 and 2 can be adjusted bymeans of an eccentric disc 13 or by hand, with greater or smaller force. To the hollow conductor 12 is secured a magnetron 14 which supplies microwave frequency energy (for example in the range 0.9 to 2.5 CH2) and which, via anexciter system 15 extending into the hollow conductor 12, feeds energy in the micro-wave range into the'hollow conductor l2 and thus into the entire hollow-conductor or waveguide system. This magnetron 14 is preferably situated centrally of the U-shaped hollow conductor 12, but may also be situated to the right or to the left up to half the length of a wave, to enable the relative degree of excitation in the two runs of hollow conductor 1 and 2 to be controlled.

The resonator 4 can be brought into resonance automatically or by hand, by means of a slide 17 accommodated in a tap line 16. In addition, a pipe connection 18 is connected to the tap line 16 via a grid 19 and enables hot air or a hot inert protective gas to be fed into the internal cavity 20 of the central resonator 4 in adjustable quantities and/or at an adjustable temperature. This protective gas leaves the apparatus through i the opposite ends of the horizontal composite waveguide. The protective gas ensures that any easily volatile and explosive substances which may form in the apparatus, for example, during the vulcanization of rubber, are rapidly evacuated therefrom, without any undesirable cooling of the objects to be treated and present therein.

Where it is desired to ensure that the protective gas does not flow through the runs of waveguides 1 and 2 the composite waveguideaccommodating the conveyor belt 6 is provided with a lining which consists of two opposed U-shaped channel members 21 and 22 fitting into each other and stretched by tension springs 23 over the outer edges of the hollow end conductors 5, thus surrounding the conveyor belt 6 running through the apparatus, as may be seen, in particular, from FIG. 3. This results in an approximately square section passage through which pass the objects 24 to be heated.

FIG. 1 indicated that in the region of the intermediate waveguides 3 lines of force pass through the objects to be heated, approximately in the transversal direction, in accordance with the arrows 25, while in the region of the runs of waveguides 1 and 2 the lines of force are oriented approximately longitudinally or the path of movement of the objects 24, in accordance with the arrows 26. This results in a repeated alternation, by in each case, in the angle of flow of energy in the objects passing through the device on the conveyor belt. This alternation of the field is important for an even heating effect, particularly in the case of complex plastics or rubber surface shapes and profiles. In the central piece 4, however, which serves as a resonance chamber, lines of force are present in both directions, as implied by the arrows 27 and 28.

By adjustment of the position of the rollers 9, as also by that of the U-shaped waveguide 12, it is possible to ensure that the objects 24 present on the conveyor belt 6 always occupy the most favorable position when passing through the horizontal composite waveguide, i.e., the position in which the vulcanization or hardening takes place under the optimum conditions, without any risk of local overheating. Normally, one of the two means of adjustment would suffice, but the rollers 9 enable a coarse adjustment to be effected, while the fine adjustment is carried out via the eccentric disc 13 and the hollow conductor 12. If the rollers 9 are positioned at different heights, so that the upper stringer of the conveyor belt 6 slants upwards or downwards as it passes through the apparatus, this may result in still more favorable operating conditions, inasmuch as different parts of the objects then move in succession into the most suitable position for treatment in each case, so that an evenly treated final product is obtained at the outlet from the apparatus.

Although, in the drawing, the runs of waveguides 1 and 2 in each case consist of only two U-sections placed together, or take the form of one single complete S-shape train, it is also possible for a number of portions of this kind to be provided in succession to one another, so that the objects may be treated a greater number of taimes. A limit is when the energy emanating-from the magnetron 14 has been completely consumed. Moreover the U-shaped hollow conductor 12 serving as a branching means for the energy can be dispensed with, if more moderate performances are required, and the excitation connection can be made direct to a run of each hollow conductor 1 or 2.

It has been found that a very even heat treatment, without local hotspots, can be effected, in apparatus as described above, even where the articles to be heated are of complex and irregular shapes. This is due to the flows of energy to which thearticles are sequentially exposed being staggered and acting at different angles to one another. The angles of the alternate energy flows are preferably from 45 to 90 to one another. The described apparatus also permits the production of the desired different energy flows with a single excitation system. Moreover the distance between the energy source and the articles can be adjusted and a number of other adjustments are provided, to permit the operating conditions to be ad justed over a wide range to suit different articles and materials, and to compensate for load variations. In suflicient energy absorption is balanced out by the waterloads.

It is also found that the described apparatus has a considerable electrical band-width and that no critical oscillation conditions occur. Finally, in the preferred apparatus, the alternate longitudinal and transverse energy flows to which an article is exposed avoid high field intensities acting constantly in one direction, which could easily cause local burning.

We claim:

1. Apparatus for the continuous dielectric heating of articles of plastics, rubber or like materials, said apparatus comprising a hollow waveguide, conveyor means extending longitudinally within said waveguide, means for moving said conveyor within said waveguide to move articles to be heated through said waveguide, at least two further waveguides communicating at an angle with said first mentioned hollow waveguide, wherein each of said further waveguides has an end portion parallel and adjacent the end portion of the other waveguide, a relatively movable connection waveguide interconnecting said two further waveguides, said relatively mova ble waveguide being U-shaped and telescopically received on said parallel and adjacent end portions, and means for exciting said waveguides with microwave energy to provide spaced energy flows therein at an angle to one another.

2. Apparatus according to claim 1 wherein said further waveguide is sinuous in shape and communicates and intersects with said first waveguide at two or more spaced apart locations.

3. Apparatus according to claim 1 wherein said exciting means comprises a magnetron, and means displaceably mounting said magnetron and said movable connecting waveguide, whereby to permit adjustment in the relative excitation on the two waveguides.

4. Apparatus according to claim 1 including a resonant chamber, and means coupling said resonant chamber in said first hollow waveguide intermediate the ends thereof.

5. Apparatus according to claim 1 wherein said conveyor comprises a conveyor belt, said conveyor belt being supported and guided in said waveguide on vertically adjustable supporting rollers.

6. Apparatus according to claim 1 including a tap line communicating with said resonant chamber and means for supplying protective gas to said tap line.

7. A method for the continuous dielectric heating of articles of plastics, rubber or like material by the use of microwave energy comprising:

a. Providing at least two waveguides intersecting one another so that microwave energy flowing there through is angularly related at such areas of intersection,

b. generating such microwave energy from a single microwave energy source associated with one of said waveguides so as to expose an article to be heated successively to at least two spaced apart even flows of microwave energy, and

c. adjustably controlling the degree of excitation of said single microwave energy source thereby to provide even heat treatment irrespective of the type of article to be heated.

8. The method of claim 7 wherein the articles are exposed to a plurality of said flows of microwave energy; lternate flows acting at one angle and the remaining flows at another angle.

9. The method of claim 7 wherein said two flowsof microwave energy act at an angle of between 45 and to one another.

10. The method of claim 7 further including the steps of moving the article to be heated in a predetermined path, said spaced apart flows of energy acting respectively substantially longitudinally of said path and substantially transversely of said path. 

1. Apparatus for the continuous dielectric heating of articles of plastics, rubber or like materials, said apparatus comprising a hollow waveguide, conveyor means extending longitudinally within said waveguide, means for moving said conveyor within said waveguide to move articles to be heated through said waveguide, at least two further waveguides communicating at an angle with said first mentioned hollow waveguide, wherein each of said further waveguides has an end portion parallel and adjacent the end portion of the other waveguide, a relatively movable connection waveguide interconnecting said two further waveguides, said relatively movable waveguide being U-shaped and telescopically received on said parallel and adjacent end portions, and means for exciting said waveguides with microwave energy to provide spaced energy flows therein at an angle to one another.
 2. Apparatus according to claim 1 wherein said further waveguide is sinuous in shape and communicates and intersects with said first waveguide at two or more spaced apart locations.
 3. Apparatus according to claim 1 wherein said exciting means comprises a magnetron, and means displaceably mounting said magnetron and said movable connecting waveguide, whereby to permit adjustment in the relative excitation on the two waveguides.
 4. Apparatus according to claim 1 including a resonant chamber, and means coupling said resonant chamber in said first hollow waveguide intermediate the ends thereof.
 5. Apparatus according to claim 1 wherein said conveyor comprises a conveyor belt, said conveyor belt being supported and guided in said waveguide on vertically adjustable supporting rollers.
 6. Apparatus according to claim 1 including a tap line communicating with said resonant chamber and means for supplying protective gas to said tap line.
 7. A method for the continuous dielectric heating of articles of plastics, rubber or like material by the use of microwave energy comprising: a. Providing at least two waveguides intersecting one another so that microwave energy flowing there through is angularly related at such areas of intersection, b. generating such microwave energy from a single microwave energy source associated with one of said waveguides so as to expose an article to be heated successively to at least two spaced apart even flows of microwave energy, and c. adjustably controlling the degree of excitation of said single microwave energy source thereby to provide even heat treatment irrespective of the type of article to be heated.
 8. The method of claim 7 wherein the articles are exposed to a plurality of said flows of microwave energy, alternate flows acting at one angle and the remaining flows at another angle.
 9. The method of claim 7 wherein said two flows of microwave energy act at an angle of between 45* and 90* to one another.
 10. The method of claim 7 further including the steps of moving the article to be heated in a predetermined path, said spaced apart flows of energy acting respectively substantially longitudinally of said path and substantially transversely of said path. 